The most common wheel suspension geometry for the steerable wheels of a vehicle is an Ackerman wherein the axes of rotation of the steerable wheels at all times intersect with one another along the axis of rotation of the non-steerable wheels. As the wheels are turned, the outer wheel must turn at a lesser angle than the inner wheel to prevent scuffing of the wheels as the vehicle makes a turn. In an Ackerman system turning of the steerable wheels results in an increase in toe out. Determining the toe-out-on-turns is one of the measurements which must be made during a typical wheel alignment operation and has been difficult even with the development of sophisticated electronic instruments. Indeed, the most advanced computerized wheel alignment systems require that the technician manually record the steering angles from the wheel turntables, record the toe at two or more different angles of the steerable wheels, and then calculate the change in toe between the two wheel positions. The TOOT angles can be measured directly with the use of electronic turntables, but at a high initial cost, the use of complex parts, and additional maintenance problems.
The manufacturers of automotive vehicles provide specifications for various alignment parameters such as camber, caster, steering axis inclination, and toe as well as TOOT. While the calculations for TOOT are given at a steering angle of twenty degrees, the specifications for caster and steering axis inclination are taken at ten degrees. As a consequence, the steerable wheels have had to be turned from the straight ahead position through precisely ten degrees to determine caster and steering axis inclination and then turned another ten degrees to determine TOOT. While time could be saved if the caster, steering axis inclination, and TOOT parameters were specified at the same steering angles, this has not been the case, and as a result, the normal wheel alignment procedure takes longer than would otherwise be the case.